1. Field of the Invention
This invention relates to a tie layer composition and to laminated structures comprising the same. More particularly, this invention relates to a tie layer composition comprising a blend of carboxylated and uncarboxylated polymer and to laminated structures comprising a plurality of polymeric layers bonded together with said tie layer composition.
2. Prior Art
Laminated structures comprising a plurality of polymeric layers are, of course, well known n the prior art. Such structures are taught, for example, in U.S. Pat. Nos. 4,058,647; 4,198,327; 4,298,712; 4,332,858; 4,341,837 and 4,588,648 and in Japanese Patent Application No. Sho 58 [1983]-13242. In general, laminated structures comprising a plurality of polymeric layers are fabricated for the purpose of obtaining a single structure having the advantages of each of the separate layers. Before such structures will perform successfully, however, it is important that the various layers be suitably adhered one to the other so as to avoid separation (delamination) during use. Heretofore, several methods have been proposed for effecting such suitable adherents of layers. For example, U.S. Pat. No. 4,198,327 teaches that various polar materials which include polycarbonates and saponification products of an ethylene/vinyl acetate copolymers may be satisfactorily bonded with a composition comprising a predominant amount of a carboxylated crystalline polyolefin and a minor amount of a hydrocarbon elastomer. Inclusion of a small amount of the hydrocarbon elastomer in the composition improved the adhesion of the polyolefin to the polar materials. Similarly U.S. Pat. No. 4,058,647 teaches that certain polymeric materials, including ethylenic/vinyl alcohol copolymers, may be laminated with a composition comprising both a modified and an unmodified polyolefin and a rubber component. U.S. Pat. No. 4,298,712 also teaches that good adhesion can be obtained with a composition comprising a modified high density polyethylene, a polyethylene resin and an elastomer. U.S. Pat. Nos. 4,332,858 and 4,341,837 teach that various polymeric materials including olefin homopolymers olefin copolymers such as ethylene and vinyl alcohol polycarbonates and the like may be adhered with a modified, but unhydrogenated, block copolymer such as a maleated block copolymer of styrene and butadiene. U.S. Pat. No. 4,588,648 teaches that certain polymeric materials such as polypropylene and ethylene/vinyl alcohol copolymers may be laminated with an adhesive layer comprising a grafted copolymer of an olefin and maleic anhydride and an ungrafted polypropylene. Japanese Pat. Application No. Sho 58 [1983]-13242 teaches that various polymeric materials such as olefin homopolymers, olefin copolymers such as ethylene/vinyl alcohol polycarbonates and the like may be tied in a laminated structure with an adhesive comprising a modified crystalline olefin-based polymer which adhesive may also comprise an unhydrogenated block copolymer of styrene and butadiene.
As is well known, many of the adhesive compositions heretofore proposed have been effective for producing laminated structures having a broad range of applications. Many of the adhesive compositions proposed heretofore, particularly those comprising modified polyolefins either alone or in combination with unmodified polyolefins and a minor amount of elastomer generally do not act as a compatibilizer for reground scrap of laminates prepared therewith. Thus direct reuse of any excess of such laminate as by way of a regrind layer in the laminate or in other applications such as in the fabrication of molded structures is not generally possible. Moreover, laminated structures comprising unsaturated elastomers are, generally, not thermally stable and cannot effectively be used when the method used to prepare the laminated structure involves temperatures at or above the temperature at which degradation of the unsaturated elastomer occurs. Further, many of the adhesive compositions heretofore proposed do not permit high temperature finishing or thermoforming or use of the laminated structure in applications requiring high temperature operations such as retorting, hot filling and the like. As a result, laminated structures prepared with such adhesive compositions cannot be used in those applications requiring high temperature finishing or use. In light of these deficiencies, then, the need for an adhesive composition which can be used to prepare laminates of various polymeric materials which produce compatible laminates and which can be prepared, finished and used without severe temperature limitations is believed to be readily apparent.
Adhesive and similar compositions which may, at least, comprise a hydrogenated block copolymer containing at least one monoalkenyl aromatic hydrocarbon block and at least one conjugated diolefin block, which adhesive compositions are useful over a broader range of temperatures, are, of course, known in the prior art. Such adhesive and similar compositions are taught, for example, in U.S. Pat. Nos. 3,607,977 and 3,970,771 and in European Patent Application No. 0169987. U.S. Pat. No. 3,607,977 teaches compositions comprising a block copolymer containing at least two monoalkenyl aromatic hydrocarbon polymer blocks and at least one conjugated diolefin polymer block and a block copolymer comprising at least one monoalkenyl aromatic hydrocarbon polymer block and at least one conjugated diolefin block wherein at least 10% of the initial unsaturation is reacted so as to incorporate a polar group. The block copolymers may, independently, be neat or hydrogenated. The polar group may, inter alia, be an oxygen containing group such as a carboxyl group. According to this patent, compositions comprising an oxygen containing group are especially useful as adhesives. particularly for polar textiles such as cellulose. U.S. Pat. No. 3,970,771 teaches a primer coating composition comprising a selectively hydrogenated block copolymer having at least two monoalkenyl aromatic hydrocarbon polymer blocks and at least one hydrogenated polymer block of a conjugated diolefin, a substantial portion of a resin compatible with the monoalkenyl aromatic hydrocarbon blocks and, optionally, a substantial portion of certain carboxylated resins. The primer is useful for bonding various coating materials to substrates having low energy surfaces such as substrates which are essentially hydrocarbon. European Patent Application No. 0169987 teaches an adhesive composition comprising a copolymer of an olefin and an ethylenically unsaturated carboxylic acid, and, optionally, an elastomer which may be a hydrogenated block copolymer of a conjugated diolefin and a vinyl aromatic hydrocarbon compound. The adhesive composition may be used to bond a polymer layer such as a polyamide or a polyester to a metallic screen. While certain of the adhesive compositions taught by these patents would be useful over a broader range of temperatures than those containing ethylenically unsaturated polymers heretofore used in the preparation of polymeric laminates, there is no indication that such adhesives would be useful in the preparation of such polymeric laminates, particularly polymeric laminates comprising polar and non-polar polymeric layers. Moreover, adhesive compositions containing a relatively large concentration of carboxyl groups have been found to be deficient when used as a tie layer in laminated structures comprising at least one layer containing polar groups which are reactive with carboxyl groups, at least when the laminate is prepared by bringing the various layers together in the molten phase while the various layers are in motion. The need, then, for an improved adhesive composition for preparing laminated structures containing at least one layer having polar groups which are reactive with carboxyl groups continues, particularly when the laminated structures are prepared by bringing the various layers together in the molten phase while the various layers are in motion.